Field of the Invention
The invention, which is also referred to as a “closed up system”, relates to a gas-tight industrial heat treating furnace into which protective gas is admitted. More particularly, the heat treating furnace is embodied as a chamber furnace, a pusher-type furnace, a rotary hearth furnace, or a ring hearth furnace for the heat treatment of batches of metal workpieces.
Description of the Related Art
Industrial furnaces that can largely be assigned to the above generic type are known. The demand for a particularly high throughput rate of such industrial furnaces is linked to very flexible production, for which reason continuously operating furnace plants have also become established, aside from automated chamber furnace plants. Single- or multi-line pusher-type furnace plants, rotary hearth furnaces and ring hearth furnaces are primarily used in continuous furnace plants.
Thus, it is known according to DE 199 47 482 B2 to undertake an optimization of the use of rotary hearth furnaces by a special arrangement or splitting-up of pallets in a rotary hearth furnace.
According to DE 102 58 728 A1, a ring hearth furnace is proposed with a hollow hub, which is suitable for the treatment of shells with bulk material.
Moreover, DE 35 06 131C1 discloses a method and a device with a time-independent batch removal from a rotary hearth furnace as well as a heating operation in a separate preheating zone.
In this regard, it was found that a time-independent removal of batches and a heating operation in a separate preheating zone is costly at least in terms of energy.
According to the generally known prior art, generic plants of industrial furnaces are in principle suitable for the standard heat treatment processes, such as carburizing, nitriding, hardening, annealing and tempering.
For performing simultaneously one or more case hardening steps with the workpieces, single- or multi-line pusher-type furnaces have become established, which are characterized by a compact and therefore space-saving design.
Continuous furnaces that operate according to the pusher principle are suitable for the heat treatment of charged parts with and without protective gas atmospheres. The high-temperature heat treatment furnace and the quenching facility usually form a unit. The latter is linked together with low-temperature furnaces for preheating and for tempering, with cleaning systems and charge transport systems, to form complex plants. The latter thus also enable the treatment of large batch weights without problem.
The equipping of a high-temperature heat treatment furnace with a powerful recirculation fan provides both a homogeneous distribution of the protective gas in the individual zones of the heat treatment chamber (e.g. heating zone, carburizing zone, diffusion zone etc.), as well as a generous flow circulation of the highly reactive gases around the workpiece. This system and the optimized thermodynamics as well as burner systems ensure an optimum uniformity and reproducibility of the heat treatment results.
Intermediate doors between the individual heat treatment zones further enable the achievement of required temperature differences and different carbon levels in the individual regions.
Apart from pusher-type furnaces, rotary hearth furnaces and ring hearth furnaces have also become established for use in continuous heat treatment plants, in order to treat large series of parts of identical design and thus to achieve high throughput rates.
By means of a process-related arrangement of different heat treatment zones beside one another in the form of various ring hearth furnaces, it is possible, here too, to achieve simultaneously different case hardening depths in the same plant. Different dwell times during the carburizing phase can be controlled individually with a corresponding configuration of the process control.
Rotary hearth furnaces are furnaces with a disc-shaped hearth and a large treatment space without zone separation. Ring hearth furnaces are furnaces which have an annular hearth and heat treatment space, wherein the zones are separated from one another by intermediate doors.
Rotary hearth furnaces which have a disc-shaped hearth are usually advisable only up to a limited size, because otherwise the furnace space and therefore the gas consumption of the furnace becomes too large. The furnaces of this type are loaded and unloaded manually or automatically and can be linked to other plant components. The rotary hearth furnace type is mainly suitable for individual batch loading and removal and is therefore used mainly for the carburizing or reheating of workpieces before a press-hardening process.
Ring hearth furnaces are suitable both for individual loading as well as for loading in the form of batches. Whereas individual loading is primarily used in connection with subsequent press hardening, loading with batches chiefly takes place with subsequent quenching in oil, salt or gas. The fundamental advantage of ring hearth furnaces consists of a central hearth drive, as a result of which a plurality of drive motors is not required, in contrast with a pusher-type furnace. The careful transport of the workpieces is also advantageous, since no relative displacement of the batch with respect to the hearth is required. This arrangement results in an extremely small mechanical loading of the batch carriers and the hearth floor, and use can be made of lighter batch carriers.
It proves to be a drawback with these generically described types of industrial furnaces, however, that the frequent door opening during the loading and unloading of batches of workpieces into the high-temperature heat treatment chambers disturbs the furnace atmosphere because the internal or excess pressure in the furnace collapses and a fall in the carbon level in the furnace atmosphere also occurs.
As a result, the treatment of the workpieces is adversely affected in terms of quality because an increased edge oxidation of the workpieces can be caused. Moreover, a large quantity of protective gas has to be repeatedly conveyed into the furnace after the closing of the doors in order to restore the desired furnace pressure and the required carbon level. Apart from the heat treatment process being disturbed, the energy loss is therefore also a disadvantage.